This invention relates generally to the field of mass spectrometry, and more particularly to sample ionization for mass spectrometer system. More particularly, this invention relates to an ionization apparatus and method for connection to a mass analyzer to improve mass analysis by seamlessly combining sample ionization and sample analysis.
Mass analysis of any sample in a mass spectrometer requires sample ionization as a first step. Sample ionization can be performed under either vacuum or atmospheric pressure. Vacuum ionization techniques include electron impact ionization, fast ion bombardment, secondary ion ionization, and matrix-assisted laser deposition/ionization. Vacuum ionization occurs inside a mass spectrometer instrument under vacuum conditions. A disadvantage of vacuum ionizations is that a sample support must be inconveniently introduced into the vacuum via vacuum locks, making the linking of mass spectrometry with chromatographic and electrophoretic separation methods more difficult.
Atmospheric pressure ionization takes place outside of a mass spectrometer instrument. To sample atmospheric pressure ions, a mass spectrometer must be equipped with an atmospheric pressure interface (APE) to transfer ions from an external region at atmospheric pressure to a mass analyzer under high vacuum. Atmospheric pressure ionization techniques include atmospheric pressure chemical ionization and electrospray ionization (ESI) among others. One problem of many prior art atmospheric pressure ionization techniques is the low transmission efficiency of sample ions to a mass analyzer due to ion losses and low throughput of ions for mass analysis due to non-seamless connection of atmospheric sample ionization and sample analysis under high vacuum.
U.S. Pat. No. 5,663,561 describes a device and method for ionizing analyte molecules at atmospheric pressure by chemical ionization. According to this method, the analyte molecules deposited together with a decomposable matrix material are first blasted in the surrounding gas under atmospheric pressure to produce neutral gas-phase analyte molecules. Then these neutral gas-phase analyte molecules are ionized by atmospheric pressure chemical ionization. This method requires that the desorption of the analyte be carried out as a separate step from the ionization of the analyte.
U.S. Pat. No. 5,965,884 describes an atmospheric pressure matrix assisted laser desorption ionization (AP-MALDI) ion source. The AP-MALDI apparatus contains an atmospheric pressure ionization chamber hosting a sample to be analyzed, a laser system outside the ionization chamber, and an interface that connects the ionization chamber to the spectrometer. While this AP-MALDI apparatus combines analyte desorption and ionization in a single step, it cannot be operated at an optimum pressure regime, and ion transmission from the ionization chamber to the spectrometer is low. Moreover, analyte adducting is high and undesired molecular clusters are formed during the ionization process.
EP 0964427 A2 describes a MALDI ion source operating at pressures greater than 0.1 torr. While the claimed ion source may be operated at a greater pressure range, it has the same problems as U.S. Pat. No. 5,965,884: low ion transmission, high adducting among analytes and other molecules and undesired cluster formation.
WO 99/38185 and U.S. Pat. No. 6,331,702 B1 describe a spectrometer provided with a pulsed ion source and transmission device to damp ion motion and method of use. This design requires a sample loading chamber or lock chamber and a low pressure MALDI ion source, and has limited throughput.
WO 00/77822 A2 describes a MALDI ion source that is enclosed in a chamber and operated under a low pressure and has a limited throughput.
Accordingly it is an object of the present invention to provide an ionization apparatus for connecting to a mass analyzer to seamlessly combine sample ionization and sample analysis.
It is another object of the present invention to provide an ionization apparatus for fast sample scanning to increase throughput of mass analysis.
It is a further object of the present invention to provide an ionization apparatus which allows sample preparation at atmospheric pressure to increase reliability and reduce construction cost of mass analysis systems.
In accordance with the invention, there is provided an ionization apparatus for connection to a mass analyzer. The ionization apparatus comprises a sample slide having at least two sample spots containing analytes to be analyzed by a mass analyzer, means for delivering energy to one of the sample spots to release and ionize the sample analytes to form sample ions, and an interface for supplying the sample ions to the mass analyzer. The interface comprises a chamber having an orifice in close proximity to the irradiated sample spot and defining a first region encompassing the irradiated sample spot. An ion guide is disposed in the chamber and leads to the mass analyzer in a second region. Means for sustaining a pressure substantially lower than atmospheric within the first region is provided for capturing the ions while other sample spots are maintained at atmospheric pressure. Means for sustaining a pressure within the second region substantially lower than the pressure within the first region is provided.
The means for delivering energy is disposed such that the energy irradiates one of the sample spot through the orifice in front of the irradiated sample spot. Alternatively, the means for delivering energy is disposed such that the energy irradiates one of the sample spots from the back of a transparent sample slide.
The ionization apparatus may comprise a motorized stage for moving the sample slide to sequentially present sample spots to the first region. The motorized stage can be computer controlled and moveable in three dimensions. The sample slide is preferably disposed in proximity of about from 50 to 100 microns to the interface.
The ionization apparatus may comprise a cover slide that seamlessly takes place of the sample slide with the same proximity to the orifice when the sample slide moves away during sample change.
The means for sustaining a pressure substantially lower than atmospheric within the first region can maintain a pressure from few torr to few tens torr. The means for sustaining a pressure within the second region can maintain a pressure from about 0.001 to about 0.1 torr.
In another embodiment of the present invention, there is provided an ionization apparatus further comprising an external groove surrounding the orifice to stabilize the pressure within the first region. This ionization apparatus may further comprise spacing balls for engaging the sample slide and the interface to accurately space the slide from the orifice.
In another aspect of the present invention, there is provided a method for ionizing analytes in a sample for mass spectrometer analysis. The method comprises providing a sample slide having at least two sample spots containing analytes to be analyzed by a mass analyzer and providing an interface connecting one of the sample spots to the analyzer. The interface is provided with a chamber having an orifice in close proximity to one of the sample spots and defining a first region encompassing the sample spot. An ion guide is disposed in the chamber leading to the mass analyzer in a second region. Energy is delivered to one of the sample spots to release and ionize the analytes to form ions. A pressure substantially lower than atmospheric is sustained within the first region while maintaining atmospheric pressure at other sample spots. A pressure within the second region substantially lower than the pressure within the first region is provided.